High speed detection of faults on multi-terminal power system transmission lines has been attempted by using digital current differential measurements. Differential techniques rely on the fact that, under normal conditions for each phase, the sum of currents entering terminals equals the charging current for that phase. In one conventional digital differential current system, the procedure is to compare individual samples or use a one-cycle window, use a conventional dual slope operate-restraint characteristic, and compensate for line charging. This system is not flexible enough to operate in both high and low bandwidth communication channels. Additionally, the sensitivity of this system is low because conventional operate-restraint characteristics are not adaptive. In another conventional digital differential current system, charges are calculated at both ends of a two terminal system by integrating respective current signals and are then compared. This system has sensitivity limitations and works only for two terminal embodiments.
Many power system monitoring, protection, and control functions could be performed more efficiently and accurately if power system digital measurements at multiple locations were synchronized. Generally such measurements are only somewhat synchronized because of difficulty in accurately synchronizing sampling clocks physically separated by large distances. Conventional uses of digital communications to synchronize sampling clocks at remote locations have accuracies limited by uncertainties in the message delivery time. In particular, digital communications can have different delays in different directions between a pair of locations which lead to an error in clock synchronization.
Variable size data windows in power system protection devices have generally been avoided because of the associated complexity, computational burden, and communications requirements. Where digital communications can have different delays in different directions between a pair of locations which lead to an error in clock synchronization.
Variable size data windows in power system protection devices have generally been avoided because of the associated complexity, computational burden, and communications requirements. Where variable size data windows have been implemented, a different set of weighting functions is used for each data window. When the data window changes size, recalculations are then required for all the samples in the data window.
Conventional power system impedance relays, including electromechanical, solid state, and digital relays, typically detect faults by calculating an effective impedance from voltage and current measurements. When the effective impedance falls within a certain range, a fault is declared. For a first zone relay, the range is typically set for less than 85-90% of the total line length impedance to allow for uncertainties in the underlying measurements of power system quantities. The actual uncertainties vary with time. Conventional impedance relays do not recognize the time varying quality of the underlying measurements, and thus sensitivity and security can be compromised.
Inherent uncertainties exist in estimating fundamental power frequency voltages and currents from digitized samples and arise from a number of sources, including, for example, power system noise, transients, sensor gain, phase and saturation error, and sampling clock error. Conventional practice is to account for these errors during system design by estimating the worst case and including enough margin to allow for the errors. The conventional procedure does not take into account the time varying nature of the errors. Other procedures to determine the sum of the squares are computationally intensive.
The standard method for providing transformer current differential protection is to develop restraint and operate signals from measured transformer currents from each winding, and to use a discrete Fourier transform (DFT) or a fast Fourier transform (FFT) to calculate various harmonics. The operate signal is usually calculated based on the principle that the sum of the ampere turns approximately equals the magnetizing current and is thus calculated as the algebraic sum of the ampere turns for each winding. The restraint signal is usually based on the fundamental frequency current or a weighted sum of the fundamental frequency current and selected harmonics to factor magnetizing inrush and overexcitation.
It would be desirable to have a digital differential current system capable of operating for a wide range of bandwidth communication channels with faster response time and increased sensitivity over conventional systems.
It would also be desirable to have methods for synchronizing power system measurements at multiple locations; for calculating the fundamental power system frequency component of voltages and currents from digital data samples over a variable size data window; for calculating uncertainties from power system quantity measurements in such a manner that a reach (the setting of a distance relay) is continuously adapted to the quality of the measurements; and for determining the uncertainty in fundamental power system frequency measurements of voltages and currents by estimating the errors on-line from available information in a way that tracks the time-varying nature of the errors.
In the present invention, current measurements are transmitted by a data consolidation of a partial sum of the terms used in a discrete Fourier transform (DFT) and the required digital communications bandwidth is thereby reduced; an adaptive restraint region is automatically adjusted using statistical principles to reflect the confidence in current measurements during changing system conditions; and sampling synchronization can be achieved by analyzing data in the measured currents.
Data consolidation involves the extraction of appropriate parameters to be transmitted from raw samples of transmission line phase currents. Data consolidation can be used to achieve a balance between transient response and bandwidth requirements. Consolidation is possible along two dimensions: time and phase. Time consolidation combines a time sequence of samples to reduce the required bandwidth. Phase consolidation combines information from the three phases and the neutral. Phase consolidation is generally not used in digital systems wherein detection of which phase is faulted is desired. Time consolidation reduces communications bandwidth requirements and improves security by eliminating the possibility of falsely interpreting a single corrupted data sample as a fault. The present invention includes a new consolidation technique called xe2x80x9cphaselets.xe2x80x9d Phaselets are partial sums of terms of a complete phasor calculation. Phaselets can be combined into phasors over any time window that is aligned with an integral number of phaselets. The number of phaselets that must be transmitted per cycle per phase is the number of samples per cycle divided by the number of samples per phaselet.
A restraint characteristic is the decision boundary between conditions that are declared to be a fault and those that are not. The present invention includes an adaptive decision process based on on-line calculation of the sources of measurement error to create an elliptical restraint region having a variable major axis, minor axis, and orientation. Parameters of the ellipse vary with time to take advantage of the accuracy of the current measurements.
With respect to synchronization, the conventional technique, as described in Mills, xe2x80x9cInternet Time Synchronization: The Network Time Protocol,xe2x80x9d IEEE Transactions on Communications, vol. 39, no. 10, October 1991, pages 1482-93, is a xe2x80x9cping-pongxe2x80x9d technique which uses round trip time tag messages to synchronize clocks which calculate the communications delays. A limitation of the ping-pong technique is that the difference between the delays in each direction between two terminals cannot be determined. The present invention includes a new technique for compensating for this uncertainty in the case of two or three terminal transmission lines by using information in the measured currents and digital communication. In this manner, measurement of magnitude and phase angle of power system voltages and currents at multiple locations can be performed on a common time reference. When four or more terminals are used, the conventional ping-pong technique is used.